How does the lighting tool get the surface normal

[Helper]

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On 07/11/2012 at 08:31, xxxxxxxx wrote:

User Information:
Cinema 4D Version:   14.0 
Platform:   Windows  ;   
Language(s) :     C++  ;

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I want my tool to have feedback similar to the lighting tool where as you drag your cursor around the viewport it shows a circle that is parallel to the surface normal of whatever is under the cursor. How does it get the normal of whatever is under the cursor?

I have a method to get the normal using ViewportSelect and then checking collision with GeRayCollider but this isn't the most efficient way, especially if I'm checking it each time the mouse is moved.